1. Field of the Invention
The present invention relates to an electrophotographic image bearer which can be used for electrophotographic image forming apparatus such as copiers, facsimile machines, laser printers and direct digital plate making machines. In addition, the present invention relates to a process cartridge and an image forming apparatus using the image bearer.
2. Discussion of the Background
Electrophotographic image forming processes typically include the following processes:
(1) charging an electrophotographic photoreceptor in a dark place (charging process);
(2) irradiating the charged photoreceptor with imagewise light to form an electrostatic latent image thereon (light irradiating process);
(3) developing the latent image with a developer including a toner mainly constituted of a colorant and a binder to form a toner image thereon (developing process);
(4) optionally transferring the toner image on an intermediate transfer medium (first transfer process);
(5) transferring the toner image onto a receiving material such as a receiving paper ((second) transfer process);
(6) heating the toner image to fix the toner image on the receiving material (fixing process); and
(7) cleaning the surface of the photoreceptor after the toner image is transferred (cleaning process).
Recently, requisites for image forming apparatus using electrophotographic image forming processes, such as electrophotographic copiers and printers are as follows:
(1) being able to produce high quality images at a high speed;
(2) being small in size; and
(3) having a long life.
The life of an image forming apparatus mainly depends on the life of the photoreceptor used therefor because the photoreceptor tends to be damaged when repeatedly suffers mechanical and chemical actions during the processes of charging, light irradiating, developing, transferring and cleaning. Mechanical actions cause photoreceptors to be abraded and hurt. Chemical actions such as oxidation reaction caused by ozone deteriorate the binder resins and charge transport materials included in the photoreceptors. In addition, as a result of chemical actions, depositions adhere on the surface of photoreceptors, and thereby image qualities deteriorate.
Since image forming apparatus are speeded up and minimized as mentioned above, the photoreceptors used therefor are also minimized. Therefore usage conditions of photoreceptors become severer and severer.
From this standpoint, there are proposals for belt photoreceptors and belt intermediate transfer materials. In order to minimize the belt photoreceptors, the peripheral length of the belt photoreceptor should be minimized. However, when the peripheral length is minimized, the surface of the belt photoreceptors frequently contacts various image forming members such as a cleaner, image developer and transferer, resulting in increase of abrasion of the photosensitive layer, and thereby the life of the photoreceptors is shortened.
As another way to minimize belt photoreceptors, image bearing members are proposed which have a construction such that the surface of the photosensitive layer side of a belt photoreceptor is pressed by a pressing member such that the photoreceptor has a U shape as illustrated in FIG. 3. However, such a belt photoreceptor (i.e., the photosensitive layer) are seriously abraded because the surface of the photosensitive layer side contacts the pressing member, and thereby the electrostatic properties of the photoreceptor are deteriorated. In addition, the photosensitive layer tends to be mechanically broken, resulting in shortage of the life of the photoreceptor.
Japanese Laid-Open Patent Publication No. (hereinafter referred to as JOP) 8-179542 discloses aprotective layer having good mechanical strength to improve the abrasion resistance of the photoreceptor. However, as a result of the present inventors"" evaluation, the photoreceptor cannot produce high quality images because the resolution of the resultant images is deteriorated, namely the resultant character images are widened.
In order to reduce abrasion of the surface of photoreceptors, methods in which the friction coefficient of the surface of photoreceptors is reduced have been proposed. However, as a result of the present inventors"" investigation, it is found that photoreceptors having a low friction coefficient do not necessarily have a good abrasion resistance, namely photoreceptors having a low friction coefficient are abraded depending on the pressing member used.
JOP8-248715 discloses an image forming apparatus in which the photoreceptor used has a friction coefficient in a specific range against the developing roller. JOP 6-118770 discloses an image forming apparatus in which the friction coefficient of the photoreceptor used is smaller than that of both end portions of the charging roller used. JOPs 9-50144 and 9-90843 have disclosed image forming apparatus in which the relationship between the friction coefficient of the surface of the photoreceptor used and the friction coefficient of the cleaner used is specified.
In addition, JOPs 6-342236, 8-202226 and 9-81001 have disclosed techniques in which a member applying a lubricant to the surface of the photoreceptor used is provided around the photoreceptor. However, needless to say, when such a member is provided in an image forming apparatus, the image forming apparatus becomes large in size (i.e., the image forming apparatus cannot be minimized).
Because of these reasons, a need exists for a belt-form photoreceptor which can be used for small image forming apparatus and which can produce images having good image qualities while having a long life and high reliability.
Accordingly, an object of the present invention is to provide an electrophotographic image bearer for small image forming apparatus, which has at least a belt-form photoreceptor and a pressing member pressing the surface of the photosensitive side of the photoreceptor and which can produce images having good image qualities while the photoreceptor has a long life and high reliability.
Briefly the object and other objects of the present invention as hereinafter will become more readily apparent can be attained by an image bearing unit including at least a belt-form electrophotographic photoreceptor including at least an electroconductive substrate and a photosensitive layer located overlying the substrate and a pressing member which presses the belt-form photoreceptor while a surface of the pressing member contacts a surface of the photosensitive layer side of the photoreceptor such that the photoreceptor has a U-form or V-form portion (hereinafter simply referred to as a U-form portion) and the pressing member is driven by (i.e., rotated together with) the belt-form photoreceptor, wherein the surface of the photosensitive layer side of the photoreceptor has a static friction coefficient less than a static friction coefficient of the surface of the pressing member.
The friction coefficient of the photosensitive layer side of the belt-form photoreceptor is preferably from 0.1 to 0.4.
The surface of the photosensitive layer side preferably has a pencil hardness of 3H or harder.
The photosensitive layer preferably includes a combination of a charge generation layer and a charge transport layer. The charge transport preferably includes a charge transport material and a binder resin, and more preferably an antioxidant is included therein. The thickness of the combination photosensitive layer is preferably from 10 xcexcm to 30 xcexcm.
The peripheral length and thickness of the belt-form photoreceptor is preferably from 100 mm to 5000 mm, and from 80 xcexcm to 160 xcexcm.
In another aspect of the present invention, an image forming apparatus is provided which includes an electrophotographic image bearer; a charger configured to charge the photoreceptor; a light irradiator configured to irradiate the photoreceptor with laser light to form an electrostatic latent image on the photoreceptor; an image developer configured to develop the latent image with a developer including a toner to form a toner image on the photoreceptor; and an image transferer configured to transfer the toner image onto a receiving material, wherein the image bearer is the electrophotographic image bearer of the present invention mentioned above.
In yet another aspect of the present invention, a process cartridge is provided which includes an image bearing unit, and at least one of a charger, an image irradiator, an image developer, an image transferer, a cleaner and a discharger, wherein the image bearer is the electrophotographic image bearer of the present invention mentioned above.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.